This invention relates to a test pattern generating apparatus which generates test patterns for testing logic circuits such as semiconductor integrated circuits.
In testing a logic circuit, test patterns are supplied to the logic circuit under test, and the outputs from the logic circuit are compared with expected patterns to determine whether the logic circuit works correctly or not. The test patterns and the expected patterns are previously stored in a memory and read out therefrom when testing a logic circuit. Because of the recent developments in semiconductor devices, logic circuits for example for amplifying the outputs of semiconductor devices have become very high in speed. In order to test such a high speed logic circuit, it is necessary to generate at high speed the test patterns to be supplied to the logic circuit.
For generating test patterns with high speed, the conventional test pattern generating apparatus employs what is called an interleave method in which test patterns are stored in a plurality of memories, and read out alternately from each of the memories and in which the read out patterns are combined in time for obtaining a high speed test pattern. Therefore, in the conventional apparatus, wherein the pattern data is first divided and stored in the plurality of memories, each memory is then accessed by addresses of different phase and the read out data from all the memories is combined in terms of time, the test patterns can be generated with a faster sequence than that of the operating speed of the memories.
However, in this conventional test pattern generating apparatus, the sequence for reading out the pattern data from a plurality of memories is fixed, so that the modification of the test patterns based on pattern data already stored in the memories is not possible. If the test pattern is to be changed, the pattern data in the same memory may have to be read out in succession, so that the time period of the test pattern is directly limited by the access time of each memory. Further, memories of very large capacity are required for generating large and complicated test patterns, such as the galloping, walking and ping-pong test patterns, wherein small loops of patterns are repeated many times.